[gmx-developers] virial contributions

H.J.Risselada H.J.Risselada at rug.nl
Mon Oct 9 11:51:49 CEST 2006

Dear GMX-devolopers

When looking to the code pull.c (gmx-3.3.1) I see e.g that 
in the umbrella sampling part a virial contribution is 
calculated from the double sum of the pull forces together 
with the pulled positions relative from equilibrium (the 
well of the potential). However, instead of the expected 
factor -0.5 a factor of +0.5 is included while in other 
parts of the code the -0.5 is included (calcvir.c).

The pressure equations implemented in gromacs is the one 
directly from litterature which however also implies the 
-0.5 factor in the virial.

It's by sure not unthinkable that I just overlook a minus 
sign or a substraction somewhere else on the code, but on 
first sight it looks to me the virial contribution of 
pulling is now substracted instead of added to the 

Can someone please explain me this ?

Physically I would expect the opposite, hence it's the 
virial contribution added to form the virial which 
represents the state of the biased ensemble.




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